Filters based on transmission line resonators are fundamental components in modem radio apparatuses. Categorized according to the frequency response, the commonest filter types are band-rejection and band-pass filters which are used to attenuate high-frequency signals on a desired frequency band (band-rejection) or outside a certain frequency band (band-pass). In addition, low-pass and high-pass filters are used. Transmission line resonators, the resonating frequencies of which determine a filter's frequency response, are usually cylindrical coil conductors, or helixes, plated grooves or holes formed in a dielectric medium, coaxial outer/inner conductor pairs or striplines formed on a board-like substrate. There are usually from two to about eight resonators in a filter. A filter is connected to the rest of the radio apparatus via input, output and control signal ports.
In many applications it is advantageous if the filter's frequency response can be altered during the operation by means of sending an electric signal to the filter. For example, in many cellular mobile phones the transmission and reception occur on a fairly narrow frequency band which may be located at various parts of a wider frequency range. Then the receiver band-pass filter, the task of which is to prevent signals other than the desired signal from entering the receiver, has to be adjusted so that the attenuation minimum in its frequency response coincides with the frequency of the desired signal. There also exist in the prior art duplex filters in telephones based on frequency duplexing, wherein the receive branch pass band is wide when the apparatus is not transmitting and narrow when the apparatus is transmitting and the powerful transmitted signal must be prevented from entering the sensitive reception parts. Naturally, it must also be possible to shift the narrow reception pass band to that particular location of the reception frequency range where the desired signal is located.
In the prior art there does not exist a simple filter that could be changed by means of an electric signal from a band-rejection filter into a low-pass filter in such a manner that the filter as a low-pass filter passes the whole previous stop band but in both cases attenuates the harmonics of the band in question. A functionally equivalent arrangement according to the prior art requires two separate filters in the radio apparatus, one of which is a band-rejection filter and the other a low-pass filter. A separate switch arrangement selects one filter at a time for use. Disadvantages of this kind of an arrangement include the need for space for separate filters and the attenuation of the high-frequency signal as it propagates through the switch arrangement.
An object of the invention is to provide a radio-frequency filter which can be converted from a band-rejection filter to a low-pass filter by means of an electric signal. Another object of the invention is that the arrangement according to the invention is easily applied to filters based on various types of resonators. Yet another object of the invention is that the convertible filter according to the invention is small in size and produces only a little amount of unwanted attenuation. A further object of the invention is that the filter according to the invention can be realized using a relatively small quantity of components.